Partial Discharge Characteristics Of Typical Defect Models In Oil-pressboard Insulation Under AC-DC Combined Voltage

China's power industry has entered a period of large units,large power grid,high voltage and high automation.Uhvdc transmission has the advantages of large capacity,long distance,high efficiency and low loss.As one of the most important equipment in HVDC transmission system,the insulation state of converter transformer plays an important role in the safe and stable operation of HVDC transmission system.The valve side winding needs to bear the compound voltage of different ac and dc components.However,the existing researches mainly focus on the ratio of 1:1 composite voltage,while there are few studies on other voltage ratios.Compound voltage is established under the paper insulation board experiment platform,based on pulse current method and constant voltage method,studied the 1:1,1:3,1:5 and the proportion of 1:7 AC-DC combined voltages needle-plate electrode model,the plate electrode surface strong/weak vertical component model,bat electrode has the oil gap and column plate electrode surface strong/weak vertical component model of partial discharge initial discharge voltage,spectra characteristics and discharge characteristic;The space electric field and charge distribution of the dc component to the needle plate with oil gap model are simulated by using the fluid dynamics drift diffusion model,the bipolar loadon transfer model and the dc component to the needle plate with oil gap model.The results show that the initial discharge voltage of the model with strong vertical component along the surface is obviously lower than that of the model with weak vertical component.PRPD spectra distribution wide midgame,discharge in the positive half cycle of the ac voltage rise more along with the peak around,and exchange the positive half cycle of bureau put the average phase close to 45°.At the ratio of 1:1 AC-DC combined voltage,there will be a concentrated discharge near the negative half cycle peak with the discharge less than the positive half cycle.Qave,Fnc and Feqave of needle plate electrode with oil gap model decrease with the increase of dc component,and Teqave and Feqa,ve increase with the increase of dc component.Qave and Feqave of the model with strong vertical component along the surface of the needle plate electrode decreased with the increase of DC component,and Teqave and Fnc increased with the increase of DC component.The Qave of the weak vertical component model of the needle plate electrode along the surface decreases with the increase of the DC component,and F,c increases with the increase of the dc component.The partial discharge process of needle-plate electrode with oil gap model can be divided into the first stage of the development of flow discharge and the second stage of the formation of self-sustaining discharge by the breakdown of oil gap and the accumulation of space charge on the surface of insulating board to form the zero field region,Qave and F,c of the oil-gap model decreased with the increase of the dc component,and Teqa,ve and F,c increased with the increase of the dc component.Qave decreases with the increase of DC component,and Fnc increases with the increase of DC component.The Qave of the weak vertical component model along the surface of the column-plate electrode decreases with the increase of the dc component,and Fnc increases with the increase of the DC component.